Synthesis, crystal structure and Hirshfeld surface analysis of di-μ2-iodido-bis[(2,2′-biquinoline-κ2 N,N′)copper(I)]

In the layer structure of di-μ2-iodido-bis[(2,2′-biquinoline-κ2 N,N′)copper(I)], π–π interactions provide conectivity within and between the layers.


Chemical context
Metal complexes with N-heterocyclic ligands find wide applications in various fields such as catalysis and medicine, among others (Delgado-Rebollo et al., 2019;Novikov et al., 2021;Fong, 2016;Artemjev et al., 2022). Copper(I) bypiridine complexes are of interest because of their structural peculiarities, cuprophilic interactions, and important photochemical properties. Therefore, bypyridine-type systems are often the ligands of choice to explore new metal complexes with potentially useful properties (Ferraro et al., 2022;Starosta et al., 2012;Vatsadze et al., 2010). 2,2 0 -Biquinoline is an important and widely employed diimine ligand. The geometry of the resulting metal derivatives depends on the ligand and counterion, the metal:ligand ratio and the solvent and synthetic conditions. Here we report the preparation and structural characterization of a copper iodide complex with 2,2 0 -biquinoline. We used Hirshfeld surface analysis to estimate the contribution of non-covalent interactions to the crystal structure.

Structural commentary
The title compound crystallizes in the centrosymmetric space group P1 with one crystallographically independent molecule in the unit cell. The molecular structure is illustrated in Fig. 1. The Cu atom is coordinated in a distorted tetrahedral geometry (Table 1) by two nitrogen atoms from the 2,2 0 -biquinoline ligands and the two 2 -bridged iodide ligands. The Cu1-I1 and Cu1 i -I1 distances [symmetry code: (i) Àx + 1, Ày, Àz + 1] are 2.5734 (2) and 2.6487 (2) Å , which are close to the distances in similar compounds (Sun et al., 2013;Starosta et al., 2012) with a substituted quinoline ligand. The Cu-N distances of 2.0930 (13) and 2.0900 (14) Å are almost equal within standard uncertainty.
The quinoline fragments in the biquinoline ligand adopt, as expected, a planar geometry. The maximum and minimum deviations of the atoms from these planes are between À0.018 (2) and 0.026 (2) Å . The angle between the quinolines described by rings 1/2 (as defined in Fig. 1) is 5.08 (9) and between 3/4 is 0.59 (8) . Then, the quinoline formed by rings 1 and 2 (ring 5) makes an angle of 7.56 (5) with the quinoline described by rings 3/4 (ring 6).

Supramolecular features
The crystal packing is shown in Fig. 2, viewed down the c axis. Molecules both within the layers and between them are connected by --stacking interactions between sixmembered rings of the quinoline rings. The --stacking interaction parameters are presented in Table 2. Ring 4, defined by N2/C18/C10-C13 in Fig. 1, participates in the shortest interactions. The contact with another ring 4, related by the symmetry operation Àx, Ày + 1, Àz + 1, is perhaps the most efficient, based on the distance, the angle between the planes, and the shift between ring centroids.

Database survey
A search in the Cambridge Structural Database (CSD, Version 5.43, update of 2022; Groom et al., 2016) showed only a few hits for bis[( 2 -halogen)-2,2 0 -biquinoline-di-copper(I)]. We only found data for compounds with substituted quinoline rings in position-4 with carboxylate fragments. All compounds crystallize in the triclinic space group P1. In IRIVIP (Vatsadze et al., 2010), n-hexyl carboxylate groups are attached to the quinoline rings at position 4. In YIJFAA, YIJFEE, and YIJFII (Sun et al., 2013), ethyl carboxylate fragments are attached, and in PAYKIL (Starosta et al., 2012), there are methyl carboxylate fragments. In IRIVIP and YIJFAA, instead of the iodine atom, as in the title structure, there are chlorine atoms; in YIJFEE, there are bromine atoms. In other structures, the copper atoms are bonded through iodine atoms.

Hirshfeld surface analysis
Crystal Explorer21 was used to calculate the Hirshfeld surfaces and two-dimensional fingerprint plots (Spackman et al., 2021). The donor-acceptor groups are visualized using a standard (high) surface resolution and d norm surfaces are mapped over a fixed colour scale from À0.0579 (red) to 1.3919 (blue) a.u., as illustrated in Fig. 3 Table 1 Selected geometric parameters (Å , ).

Figure 2
View along the c axis of the crystal packing of the title compound, showing the stacking of layers formed by the Cu complex.

Figure 1
Molecular structure of the title compound, including atom and ring labelling. Displacement ellipsoids are drawn at the 50% probability level.
[Symmetry code: (i) Àx + 1, Ày, Àz + 1.] correspond to CÁ Á ÁC and IÁ Á ÁH interactions. The presence of -stacking interactions is confirmed by the characteristic red and blue triangles on the shape-index surface [ Fig. 3(b)]. Fingerprint plots of the most important non-covalent interactions for the title compound are shown in Fig. 4

Synthesis and crystallization
The title compound was prepared by refluxing CuI with one equivalent of 2,2 0 -biquinoline in ethanol for 24 h.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were placed at calculated positions (C-H = 0.95 Å ) and refined using a riding model with [U iso (H) = 1.2U eq (C)].